Add some helpful flake8 plugins

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Add some helpful flake8 plugins to the codebase:

• flake8-bugbear to find bugs/design flaws
• flake8-comprehensions to simplify list/set/dict comprehensions
• flake8-simplify to simplify over-complex code

Update code to comply with the new checks.

Raw list of suggestions:

• flake8-bugbear:

deptry/module.py:51:68: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.
deptry/module.py:51:109: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.

• flake8-comprehensions:

deptry/import_parser.py:26:26: C414 Unnecessary list call within sorted().
deptry/import_parser.py:37:23: C414 Unnecessary list call within sorted().

• flake8-simplify:

deptry/dependency_getter/pyproject_toml.py:28:16: SIM201 Use 'dep != "python"' instead of 'not dep == "python"'
deptry/dependency_getter/pyproject_toml.py:52:9: SIM105 Use 'contextlib.suppress(KeyError)'
deptry/dependency_getter/pyproject_toml.py:56:9: SIM105 Use 'contextlib.suppress(KeyError)'
deptry/dependency_getter/pyproject_toml.py:71:39: SIM118 Use '"optional" in spec' instead of '"optional" in spec.keys()'
deptry/dependency_getter/pyproject_toml.py:78:39: SIM118 Use '"python" in spec' instead of '"python" in spec.keys()'
deptry/dependency_getter/pyproject_toml.py:78:67: SIM118 Use '"version" in spec' instead of '"version" in spec.keys()'
deptry/issue_finders/misplaced_dev.py:39:13: SIM102 Use a single if-statement instead of nested if-statements
deptry/import_parser.py:87:18: SIM101 Multiple isinstance-calls which can be merged into a single call for variable 'node'
deptry/import_parser.py:97:13: SIM102 Use a single if-statement instead of nested if-statements
deptry/import_parser.py:120:60: SIM201 Use 'module != exception' instead of 'not module == exception'
deptry/import_parser.py:125:19: SIM115 Use context handler for opening files
deptry/dependency_getter/requirements_txt.py:65:59: SIM907 Use 'Optional[Dependency]' instead of 'Union[Dependency, None]'
deptry/dependency_getter/requirements_txt.py:80:54: SIM907 Use 'Optional[str]' instead of 'Union[str, None]'
deptry/dependency_getter/requirements_txt.py:102:16: SIM210 Use 'bool(re.findall('\\[([a-zA-Z0-9-]+?)\\]', line))' instead of 'True if re.findall('\\[([a-zA-Z0-9-]+?)\\]', line) else False'
deptry/dependency_getter/requirements_txt.py:106:16: SIM210 Use 'bool(';' in line)' instead of 'True if ';' in line else False'
deptry/dependency_getter/requirements_txt.py:119:46: SIM907 Use 'Optional[str]' instead of 'Union[str, None]'

[fpgmaas]

Thanks for the contribution, looks good to me!